With the development of aerospace technology, the attitude control technology with high accuracy has become a critical technology for certain spacecrafts to realize their functions, for example, communication satellites, remote sensing satellites, space telescopes, etc. Therefore, the issue of high-accuracy attitude control of a spacecraft has become a hot and difficult point in researches recently. As for an inflexible satellite, its kinetic model is relatively simple, and the corresponding control method is relatively mature. However, in recent years, a flexible spacecraft, especially a spacecraft with a large appendage has become an important direction for the development of aerospace technology in the future, for example the ETS-VIII satellite in Japan. These large flexible satellites carry light flexible appendages such as large expandable antenna arrays and solar panels, and the kinetic model of the spacecraft becomes very complicated since it is a typical non-linear, multi-coupled distribution parameter system with infinite degree of freedom. This brings along a great challenge to the high-accuracy attitude control of the spacecraft. In addition, these large flexible appendages tend to produce elastic vibration and flexible appendages with large area increase the influence of environmental disturbances such as aerodynamic drag, solar radiation pressure and the like. These vibrations and external disturbances further increase the difficulty of attitude control of the spacecraft. Accordingly, am anti-disturbance control method with a high accuracy becomes a bottleneck technology for attitude control of a large flexible spacecraft.
As for the problem of attitude control of a flexible spacecraft, different control methods are also proposed in order to offset or restrain the influence of flexible vibration and external disturbance, and the typical ones include H∞ control, self-adaptation control, sliding mode variable structure control and so on. However, most of these control methods do not have typical disturbance offset ability, and thus cause the control accuracy to be limited. Based on the kinetic model of the system, flexible vibration and environmental disturbance can be described with referenced to the external system. However, due to the error of measurement and the difference between the space environment and ground environment, the damping and frequency parameters of the flexible appendage measured in the ground experiments always have a big uncertainty, which causes the external system for describing the disturbance to be a mathematical model with uncertain parameters. Internal mode control, active disturbance rejection control (ADRC) and disturbance observer-based control (DOBC) are relatively typical disturbance compensation methods. However, the traditional internal mode control has a high requirement on the disturbance model, and requires the disturbance model to be accurately known. ADRC estimates disturbance by way of an expansion state observer without the use of the intrinsic information of the disturbance itself, and thus has certain conservatism. DOBC makes full use of the information of disturbance, estimates and compensates for the disturbance that can be modeled in the disturbance system, and achieves an ideal effect, and it can also allow the disturbance model to have certain uncertainty. However, the rate of convergence of observation error of the traditional DOBC cannot be guaranteed, and gain scheduling is very complicated, while a sliding mode observer is superior in being not sensitive to parameter change and disturbance and has a high rate of convergence. Therefore, the advantages of the traditional DOBC and the sliding mode observer can be combined by using a sliding mode disturbance observer for estimating disturbance. In this way, the disturbance model is utilized, meanwhile this method has a strong robustness to the change of the disturbance model. Furthermore, gain scheduling is very easy, and it can be ensured that the observation error can be converged into a certain adjustable area rapidly, thereby improving the accuracy, robustness and rapidity of disturbance estimation.
Therefore, there is a need for a method for disturbance compensation based on a sliding mode disturbance observer for a spacecraft with a large flexible appendage that can efficiently estimate flexible vibration and environmental disturbance.